This invention relates to sulphonated fluorocarbon humidity sensors and more particularly, to a control circuit for household or commercial appliances using such a sensor.
The use of sulphonated fluorocarbon sensors to measure humidity is known in the art. See, for example, U.S. Pat. No. 4,662,220 to Laue; or NASA Tech Briefs, Winter 1985, pages 55-56. One type sensor utilizes a sulphonated fluorocarbon manufactured by the E.I. DuPont du Nemours & Company under their trade name "Nafion". This material is a water-absorbent polymer typically delivered by the supplier in its hydrogen (H+) state. Humidity sensors using Nafion have household and commercial applications in, for example, humidifiers, dehumidifiers and air-conditioners. For these uses, the Nafion is converted from its H+ to its Na+ (sodium) state. This is done because the material in this latter state is less adversely affected by household chemicals.
Generally, humidity sensors using Nafion are incorporated as part of a control circuit for the appliance in which the control is used. While Nafion sensors work well in many appliances, there are problems with their wide spread use. It has been found that when used in high-humidity environments, the Nafion material produces leakage currents. As the voltage across the sensor increases, the leakage current begins to predominate and the Nafion starts acting as a battery. If the Nafion is placed between a stainless steel screen, for example, to create the humidity sensing element, metal from the screen begins to migrate into the Nafion, reducing its effectiveness as a sensing material. In some applications therefore, where high voltages are used, the sensor has gold plated screens or wires to prevent metal migration. While this may alleviate the problem, it is an expensive solution. Instead of having to incur this expense, it would useful and advantageous to have a lower cost alternative to reduce the above described effects of high voltage on the sensor in high-humidity environments.
A humidifier employing such a sensor also has other performance criteria which should be met. For example, it should only be operable when the unit has sufficient water. Otherwise, the unit not only does not perform its function; but also, the humidifier fan and associated components are needlessly run. And, even if the unit has sufficient water, if the wicking system of the unit is not functioning, the humidifier still will not operate as it should.